msm: camera: Fix possible NULL pointer dereference.

The result of memory allocation in msm_isp_get_buf is not checked for
success before use. This can lead to NULL pointer dereference.

Change-Id: I83cb56bbcd5e7526bf57a6de1077a30f9a2b5349
Signed-off-by: Petar Sivenov <psiven@codeaurora.org>